The lithographic printing technique makes the best use of the properties of water and oil such that they are essentially incompatible with each other. Two different areas are formed on the printing surface of a lithographic printing plate, one receiving water and repelling oil ink and the other repelling water and accepting the oil ink. The former serves as a non-image area and the latter becomes an image area.
The non-image area is made damp with a dampening water for use in lithographic printing in practice, thereby increasing the difference in surface chemical properties between the image area and the non-image area. Both the ink repellency of the non-image area and the ink receptivity of the image area can be thus increased.
The conventionally known dampening water are aqueous solutions of, for example, alkali metal salts or ammonium salts of bichromic acid, phosphoric acid or salts thereof such as ammonium salt, and a colloidal substance such as gum arabic or carboxymethyl cellulose (CMC). However, it is difficult to uniformly dampen all the non-image areas formed on the lithographic printing plate with the above-mentioned conventional dampening water, and for this reason, the resultant printed matters sometimes get smudged. In addition, a high degree of skill is required to control the amount of dampening water supplied to the printing plate. Further, recently, there have strict regulations on discharge of chromium ions into a wastewater, and the use of chromic compound tends to be regulated from the viewpoint of safety and health matter.
To overcome such problems, the Dahlgren dampening system has been introduced and widely used, which employs as a dampening water an aqueous solution containing about 20 to 25% of isopropyl alcohol. This method provides some advantages, for example, an improvement in workability and an increase in the accuracy of obtained printed matters. To be more specific, the wettability of the non-image area with the damping water is improved so that the amount of the dampening water to be applied can be reduced. The result is that the balance between the amount of the printing ink and that of the dampening water to be supplied can be easily controlled, and that the amount of water to be emulsified into the printing ink can be lowered, thereby improving the transfer performance of the printing ink to the blanket.
However, isopropyl alcohol used in the Dahlgren dampening system evaporates easily and quickly, differently from water, and therefore, a special device for keeping the predetermined concentration of isopropyl alcohol in the dampening water is required. This is unfavorable from the viewpoint of cost. Moreover, the Dahlgren dampening system is disadvantageous in terms of working environment because isopropyl alcohol gives out a disagreeable smell and produces a problem of toxicity. Isopropyl alcohol is classified into the second group of organic solvents according to Ordinance on the Prevention of Organic Solvent Poisoning, and isopropyl alcohol is now regulated. Isopropyl alcohol is also one of alcohols which are classified into the forth category in hazardous substances, being an ignitable substance, and therefore it is necessary to be careful in the handling and storage management thereof, and being unfavorable in working conditions.
Further, even if such a dampening water containing isopropyl alcohol is applied to a conventional offset printing in which a dampening molleton roller is used, no sufficient effect can be obtained because isopropyl alcohol easily evaporates from the roller surface and the printing plate surface.
Accordingly, a technology for replacement of isopropyl alcohol in a dampening water has been recently proposed, as disclosed in, for example, Japanese Patent Application Publication (hereunder referred to as “JP KOKAI”) Nos. Hei 5-92677, Hei 5-318958, 2001-287476, Hei 2-269094, Hei 3-63187, Hei 3-90389, Hei 3-90390, Hei 4-363297, Hei 5-112085, Hei 11-78281, Hei 11-105449, 2001-130164, 2001-138659, 2001-180146, 2001-18553, 2001-71658, 2002-187375, 2002-187376, 2002-192853, Sho 51-72507 and 2002-254852. However, these substitutional techniques use a volatile organic solvent to cause a discharge of VOC (volatile organic compounds), and it is desirable to improve these techniques.
On the other hand, it has been proposed to use an involatile or high boiling compound instead of isopropyl alcohol. For example, J.P. KOKAI No. Sho 51-72507 proposes that a specific alkyleneoxide type-nonionic surfactant is included in a dampening water composition, and J.P. KOKAI No. 2002-254852 proposes that an ethylene oxide and propylene oxide adduct of alkylenediamine is included in a dampening water composition. However, in case that such a dampening water composition is used, when the dampening water that remains on a printing plate surface during stop of a printing machine, becomes in the form of droplet, the water content thereof being evaporated, the involatile or high boiling compound present therein comes to remain in a concentrated condition, and then the compound is liable to exhibit a drawback such that the compound dissolves an image area in the lithographic printing plate and then impairs the image.